Massive inner refactoring

Got rid of Internal interfaces for entities
Fixed nasty bug with dynamic loading of Pjsip.Interop assembly
disposing a call after hangup
a small fix to message builder to make it easier to use
gracefully terminating in finalizer thread
added output for dtmf event in console application
refactored logging initaillization code.

logging is to be confogured explicitly by means of .config file.
Handler for NAT detected event
Fast public STUN server in config by default
ICE enabled by deafult
An example of STUN server configuration commented in .config file